Classic analgesics have limited utility because of lack of efficacy or a high incidence of side effects. Data from clinical studies and conventional clinical wisdom indicate that NSAIDs are poorly effective. Opioids may be effective but side effects, tolerance, concern about addiction and diversion all limit their utility. A review analyzing the controlled clinical data for peripheral neuropathic pain (PNP) (Kingery, Pain, 73(2):123-39 (1997) reported that NSAIDs were probably ineffective as analgesics for PNP and that there was no long-term data supporting the analgesic effectiveness of any drug. The results of published trials and clinical experience provide the foundation for specific recommendations for first-line treatments, which include gabapentin, 5% lidocaine patch, opioid analgesics, tramadol hydrochloride, and tricyclic antidepressants (reviewed by Vadalouca, et al., Ann NY Acad Sci., 1088:164-86 (2006).
Delivery of drugs by the transdermal route has been known for many years. Controlled release transdermal devices rely for their effect on delivery of a known flux of drug to the skin for a prolonged period of time, generally a day, several days, or a week. Two mechanisms are used to regulate the drug flux: either the drug is contained within a drug reservoir, which is separated from the skin of the wearer by a synthetic membrane, through which the drug diffuses; or the drug is held dissolved or suspended in a polymer matrix, through which the drug diffuses to the skin. Devices incorporating a reservoir will deliver a steady drug flux across the membrane as long as excess undissolved drug remains in the reservoir; matrix or monolithic devices are typically characterized by a falling drug flux with time, as the matrix layers closer to the skin are depleted of drug. Methods for making transdermal patches are described in U.S. Pat. Nos. 6,461,644, 6,676,961, 5,985,311, 5,948,433. Lidocaine-containing formulations are described in U.S. Pat. Nos. 4,777,046, 5,958,446, 5,719,197, 5,686,099, 5,656,286, 5,474,783, 5,300,291, 4,994,267, 4,814,168, 7,018,647, 6,299,902; and 6,297,290. U.S. Patent Application No. 2009/0048296 describes formulations containing a high concentration of lidocaine in the range of at least 20%, preferably about 40%.
Topical gels, plasters, and patches are described in U.S. Pat. Nos. 5,411,738, 5,601,838, 5,709,869 and 5,827,829 which are assigned to Endo Pharmaceuticals. The gels described in these patents contain from 2-20% lidocaine, preferably from 1-10% or 5-10% lidocaine.
A 5% lidocaine patch marketed as LIDODERM® is available from Endo Pharmaceuticals, Inc. The LIDODERM® patch comprises an adhesive material containing 5% lidocaine, which is applied to a non-woven polyester felt backing and covered with a polyethylene terephthalate (PET) film release liner. This patch is applied only once for up to 12 hours in a given 24 hour period. The marketed patch provides satisfactory therapy to some patients. Delivery of lidocaine in a patch, however, has numerous liabilities for the patient. Since the patch is a finite size and shape, the application area is determined by the patch and not by the dimensions of the painful site. If the area of pain is other than a large smooth surface, the patch may not necessarily fit the area or be comfortable to the wearer since the patch may not conform to the defect. For example, the patch is difficult to apply to toes and fingers. Applying the patch to the face creates a stigma issue for patients. The patch is undesirable for hair bearing areas as well since hair limits adhesiveness and because of the depillitation that may occur with removal of the patch. The patch may also make the patient warmer, and thus be a burden in hot environments.
The delivery of drug from the lidocaine patch is designed to be constant over the 12-hour exposure period. However, it may be therapeutically important to provide a loading dose of drug to eliminate pain quickly when first administering the therapy. It is well known in the treatment of pain that more analgesic is required to treat established pain than is needed to prevent pain from becoming more intense. Such a profile cannot be provided by a patch delivering at a constant rate.
It is therefore an object of the present invention to provide topical anesthetic formulations that can be used to provide relief from pain over a period of time.